The present invention relates to a new and improved method of reducing the chemical oxygen requirement or demand by catalytic oxidation in water which is charged with oxidizable substances. The invention further pertains to the use of the method, particularly for producing potable or drinkable water.
It is known to the art that organic and inorganic contaminants in waste water can be decomposed or otherwise degraded by chemical oxidation.
The state-of-the-art exemplifying this technology has been documented, by way of example, in the following patents and publications:
U.S. Pat. No. 3,442,802 PA1 U.S. Pat. No. 3,487,016 PA1 Journal WPCF, 48, 920 (1976) PA1 Journal WPCF, 45, 221 (1973) and so forth.
There has been described the use of manganese dioxide at room or elevated temperatures, at atmospheric or increased pressure. Yet, these techniques are more concerned with absorption of the substances at the so-called catalyst which then must consequently experience a special regeneration through the use of oxygen at elevated temperatures, amounting to as much as 300.degree. C. A similar mechanism is employed for the so-called catalysis with activated carbon.
Generally, it has been observed that most contaminants contained in water can only be oxidized extremely slowly and incompletely at ambient temperature by means of molecular oxygen. The heretofore recommended catalysts do not change anything as concerns this basic fact. The use of elevated temperatures and pressures up to and exceeding 100 bar can hardly be practically realized, particularly owing to the poor economies which prevent such application in practice, and especially when there are processed larger quantities of water.
The use of more strongly oxidizing agents than the molecular oxygen is obvious. Attempts have been made to use hydrogen peroxide, chromate and other acids, potassium permanganate, ozone and so forth alone and in combination with catalysts. Also it has been attempted to carry out chloro-oxidation alone or by means of radiation. All of these techniques, at the very best, are suitable for waste water which is only slightly contaminated with oxidizable substances. As soon as the waste load becomes greater they no longer are tolerable techniques owing to the involved operating costs.
The use of gaseous oxygen or oxygen containing-gas mixtures, for instance atmospheric air, is freely available in contrast to such oxidizing agents, and furthermore, has the advantage that any over dosage does not exert any adverse effect upon the water.
To a certain degree oxygen is a bi-radical and as such possesses a somewhat increased reaction capability. Therefore, a whole spate of organic compounds can be directly oxidized by oxygen, generally while forming peroxides or hydroperoxides. The speed of oxidation is not only dependent upon the time needed to form the peroxide, rather also upon the decomposition speed. Generally, such auto-oxidation is too slow for technical processes. As already mentioned, it is necessary to work with elevated pressures and/or temperatures in order to appropriately increase the reaction speed.
The attack of the oxygen predominantly occurs by radical or ionic splitting of C--H and C--metal--C--C--bonds or also by the addition at double bonds. In order to initiate the reaction quickly there are required initiators which initially deliver the desired or required incipient or starting concentration of radicals, in order to start the chain reaction of the oxidative decomposition. The reaction which has once been initiated can be, however, stopped by discontinuance reactions of the type: EQU R. and R..fwdarw.R--R (wherein R.=a radical).
Also the antioxidants, known as inhibitors, can prevent progress of the reaction. Therefore, it is of extreme importance that measures be provided to insure that there are always present a sufficient number of free radicals. The further course of the reaction can be controlled with suitable catalysts.